Acyl compounds addition-elimination reaction

The addition-elimination reaction of hetero-atom-substituted nitroalkenes provides functionalized derivatives of unsaturated nitro compounds.26 Nitroenamines are generally prepared from a-nitro ketones and amines (see Chapter 5 regarding acylation of nitro compounds).26... 

Moving from rearrangements, condensation reactions were also presented. Condensation reactions occur when two reactive species condense with one another forming a new compound. The first was the aldol condensation (Scheme 8.9). Later, a more complex application of the aldol condensation was presented in the form of the Robinson annula-tion (Scheme 8.10). For both of these reactions, the underlying lessons relate to the ability to induce reactions and incorporate substitutions at carbon atoms adjacent to carbonyl groups. Similar reactivities of such carbon atoms can be utilized for alkylation (SN2) and acylation (addition-elimination) reactions as illustrated in Scheme 8.11. 

Carbonyl Compounds II Nucleophilic Acyl Addition, Nucleophilic Acyl Substitution, and Nucleophilic Addition-Elimination Reactions of a, 6-Unsaturated Carbonyl Compounds... 

As we begin now to explore the syntheses of carboxylic acid derivatives, we shall find that in many instances one acid derivative can be synthesized through a nucleophilic addition—elimination reaction of another. The order of reactivities that we have presented gives us a clue as to which syntheses are practical and which are not. In general, less reactive acyl compounds can be synthesissed from more reactive ones, but the reverse is usually difficult and, when possible, requires special reagents. 

Acyl transfer reactions (Section 17.4) A reaction in which a new acyl compound is formed by a nucleophilic addition-elimination reaction at a carbonyl carbon bearing a leaving group. 

Many nucleophiles are effective in the addition—elimination reaction of acid chlorides, and a great many acyl compounds can he made using acid chlorides as starting materials. 

Acyl halides undergo addition-elimination reactions in which nucleophiles displace the halide leaving group. These compounds are so reactive that catalysts are usually not necessary for their conversion. 

In unsaturated aliphatic systems the most important reactions are those of czirbonyl compounds of the type —COX, in which X is a good leaving group such as halogen. In general, most displacement reactions of anionic nucleophiles on the carbonyl carbon atom of acyl halides involve an addition-elimination mechanism " (e.g. equation 16). In such reactions bond-formation is in advance of bond-rupture... 

The second reaction is free of metal catalysis. It takes place in the presence of acetyl chloride, proceeds faster, and results in compounds which have the same core structure of indolyl imidazoles, but with the loss of the A-oxide group The imidazole ring is activated for a nucleophilic attack by acylation of the Al-oxide function, and the reaction proceeds according to addition-elimination protocol (Scheme 71) [212]. 

As always, we need to be able to generalize. This chapter describes the chemistry of a set of related acyl compounds. It will be very hard to memorize the vast array of addition-elimination processes, but it should be relatively easy to see them all as the same reaction repeated over and over. Only the detailed stmcture changes—the overall reaction does not. 

As mentioned before, all acyl compounds participate in the addition-elimination process. Acid chlorides are especially reactive toward nucleophiles. Their carbonyl groups, being the least stabilized by resonance, have the highest energy and are the most reactive. So, an initial addition reaction with a nucleophile is relatively easy. The chloride atom of acid chlorides is an excellent leaving group, and sits poised, ready to depart once the tetrahedral intermediate has been formed... 

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